1. Field of the Invention
The present invention relates to a II-VI group compound semiconductor device, and a method for manufacturing the same. More particularly, it relates to a II-VI group compound semiconductor device having an electrode structure which shows small contact resistance, and a method for manufacturing the same. Especially, it relates to a II-VI group compound semiconductor device having an electrode layer which enables an ohmic contact between an electrode and a semiconductor layer, and a method for manufacturing the same.
2. Description of the Related Arts
So far, various types of electrode structures for a II-VI group compound semiconductor device have been studied. Haase et al., for example, have examined the applicability of Li, Na, Mg, Ti, Cr, Mn, Ni, Pd, Pt, Cu, Ag, Zn, Hg, Al, In, Sn, Pb, Sb, or Bi and alloys thereof as electrode materials ("Short wavelength II-VI laser diodes", Inst. Phys. Conf. Ser. No.120 P.9). However, electrode materials which provide an ohmic contact for II-VI group compound semiconductors have not yet been found.
Thus, Au is extensively used as an electrode metal, but Au does not form an ohmic contact, because it rather forms a Schottky junction with approximately 1.2 eV of potential barrier to p-type ZnSe.
In order to provide an ohmic contact to, for example, p-type ZnSe, methods such as the following are considered: a low contact-energy barrier intermediate layer of CdSe or HgSe is epitaxially grown between the electrode and the p-type ZnSe; or p-type ZnTe is used for a contact layer, and an intermediate layer of a p-type ZnSeTe graded composition layer or a p-type ZnSe/ZnTe strained-layer superlattice is used between the p-type ZnSe and p-type ZnTe. Ohtsuka et al. have demonstrated an ohmic contact of Au/p-CdSe and reported the possibility of an ohmic contact of Au/p-CdSe/p-ZnSe ("Growth and characterization of p-type CdSe", Ohtsuka et al., Extended Abstracts (the 54th) p.255, The Japan Society of Applied Physics). Lansari et al. have made a good ohmic contact by growing HgSe on the p-type ZnSe as a low contact-energy barrier intermediate layer by MBE method and using Au as an electrode metal ("Improved ohmic contact for p-type ZnSe and related p-on-n diode", Y. Lansari et al., Appl. Phys. Lett. 61 p.2554). Fan et al. ("Graded bandgap ohmic contact to p-ZnSe", Y. Fan et al., Appl. Phys. Lett. 61 p.3160), and Hiei et al. ("Ohmic contact to p-type ZnSe using ZnTe/ZnSe multiquantum wells", F. Hiei et al., Electronics Lett. 29 p.878) have reported the fabrication of an ohmic contact by using p-type ZnTe for the contact layer and using an intermediate layer of a p-type ZnSeTe graded composition layer or a p-type ZnSe/ZnTe strained-layer superlattice between the p-type ZnSe and p-type ZnTe.
Further, Lim et al. have made an ohmic contact by diffusing Li.sub.3 N ("Highly conductive p-type ZnSe formation using Li.sub.3 N diffusion", S. W. Lim et al., Extended Abstracts of SSDM, 1994 p.967).
However, none of the methods of making ohmic contacts to the conventional II-VI group compound semiconductors are satisfactory enough. For example, they have problems such as the following.
When CdSe is used, a low acceptor concentration of 1.times.10.sup.17 cm.sup.-3 in CdSe makes it difficult to lower the contact resistance. When HgSe is used, the sharing of the MBE apparatus, for example, used for forming other layers brings deteriorated properties of devices because of the mixing of Hg atoms into other layers. Introducing an exclusive MBE apparatus to grow HgSe leads to lower productivity. Furthermore, HgSe has poor chemical and physical stability.
When ZnTe is used, the stress remaining in the film because of a large lattice mismatch between ZnSe and ZnTe may deteriorate the properties of the devices, and it is difficult to optimize ZnTe carrier concentration. A large lattice mismatch between ZnSe and any of the above intermediate layers also causes strain, and the epitaxial growth lowers the productivity.
When Li.sub.3 N is diffused, diffusion temperature is as high as 470.degree. C., so that, when this method is applied to the device structure, it may deteriorate the device properties and, since Li atoms are extremely liable to diffuse, it causes deterioration of the device properties in the course of time.
Furthermore, the Au electrode used in the above methods is inferior in mechanical strength such as adhesion.
Accordingly, research was continued to create a new electrode structure which makes an ohmic contact to II-VI group compound semiconductors, especially to p-type Zn.sub.X Mg.sub.1-X S.sub.Y Se.sub.1-Y (0.ltoreq.X.ltoreq.1, 0.ltoreq.Y.ltoreq.1) semiconductors.
FIG. 8 shows how the contact resistance of a metal/p-ZnSe Schottky junction depends on ionized impurity concentration with the potential barrier .phi..sub.B between the metal and the p-type ZnSe as a parameter. FIG. 7 is a band diagram illustrating the Schottky barrier width (W) at the contact interface of the metal and the p-type ZnSe. .phi..sub.B is given by the formula: .phi..sub.B =.chi..sub.s +E.sub.g -.phi..sub.M, in which .chi..sub.s represents an electron affinity of semiconductor, E.sub.g represents a bandgap of semiconductor and .phi..sub.M represents a work function of metal. FIG. 10 shows these relationships. FIG. 8 shows what is obtained by a calculation using Yu's model in which the thermoemission tunneling current is considered ("Electron Tunneling and Contact Resistance of Metal-Si Contact Barrier", A. Y. C. Yu, Solid State Electronics Vol.13, p.239 (1970)). As a result, it is found that the contact resistance decreases with increase of the ionized impurity concentration. This is due to the decrease in Schottky barrier width (W) shown in FIG. 7 with increasing ionized impurity concentration, which results in rapid increase of the tunneling current.
This is also the same in the case of a metal/p-type Zn.sub.X Mg.sub.1-X S.sub.Y Se.sub.1-Y (0.ltoreq.X.ltoreq.1, 0.ltoreq.Y.ltoreq.1) interface, a metal/intermediate layer interface or an intermediate layer/p-type Zn.sub.X Mg.sub.1-X S.sub.Y Se.sub.1-Y (0.ltoreq.X.ltoreq.1, 0.ltoreq.Y.ltoreq.1) interface. For example, drawing a figure corresponding to FIG. 8 shows a similar tendency in which the contact resistance differs by one figure against the same potential barrier parameter.
In other words, an ohmic contact can be obtained by using a intermediate layer having a high ionized impurity concentration on the p-type Zn.sub.X Mg.sub.1-X S.sub.Y Se.sub.1-Y (0.ltoreq.X.ltoreq.1, 0.ltoreq.Y.ltoreq.1) semiconductor layer surface, on which a metal electrode is then formed.
However, p-type Zn.sub.X Mg.sub.1-X S.sub.Y Se.sub.1-Y (0.ltoreq.X.ltoreq.1, 0.ltoreq.Y.ltoreq.1) semiconductor layer can be formed only by MBE method, and its ionized impurity concentration is, at best, in the order of 10.sup.17 cm.sup.-3, so that it was impossible to form a layer with a high ionized impurity concentration sufficient to make an ohmic contact.
Further, in the Japanese Unexamined Patent Publication No. HEI 5(1993)-259509, the intermediate layer is restricted to ZnCdSe and ZnHgSe and, besides, the method of forming the intermediate layer includes depositing by MBE method, which causes low productivity.